Small dirt mounds across a lawn are common evidence of biological activity occurring just beneath the turf. These piles of soil are byproducts of various organisms digging, foraging, or nesting underground. Correctly identifying the source of these mounds is the first step in understanding the subterranean landscape of a yard. Knowing the distinct calling cards left by different creatures is key to determining what is happening in the soil.
Differentiation of Subterranean Mammal and Worm Mounds
The most conspicuous mounds are those created by moles and earthworms. Mole hills are typically large, conical, and irregularly shaped piles of soil. These mounds are formed when the mole pushes excavated dirt up through a vertical tunnel to the surface. The soil is often coarse, having been brought up from deeper levels of the ground, and the mound usually lacks a visible entrance hole in the center.
In contrast, earthworm castings are granular piles of fine, processed soil. These mounds are essentially the earthworm’s feces, deposited on the surface after the animal has ingested soil and organic matter. Castings are concentrated piles of nutrient-rich material, indicating activity near the soil surface, especially during cool, moist weather. Moles produce mounds of excavated subsoil, while earthworms create mounds of fine, digested topsoil.
Mounds Created by Insects and Other Burrowers
Other organisms, primarily insects and crustaceans, create mounds with unique physical signatures that distinguish them from mammals and worms. Ants construct anthills that are small, centralized piles made of very fine soil particles, representing the material excavated from their complex underground tunnel systems. These structures serve as protection and ventilation for the colony below the surface.
Cicada killer wasps leave behind a distinct volcano-shaped mound with a central opening. The female wasp digs a burrow up to 20 inches deep in loose or sandy soil, pushing the excavated dirt out to form a prominent collar around the entrance. Burrowing crayfish, particularly in moist or poorly drained areas, construct highly recognizable “chimneys” made of stacked mud pellets. These chimneys can be several inches high around the burrow entrance, which extends down to the water table.
Ecological Role of Lawn Burrowers
While their mounds may be seen as a nuisance, these burrowing organisms play a role in the health of the soil ecosystem. Moles are primarily insectivores, and their tunneling activity aerates the soil, improving drainage and water infiltration. The soil they bring to the surface through digging is known as bioturbation, which mixes organic matter and nutrients vertically within the soil profile.
Earthworms drive nutrient cycling by consuming dead organic matter and excreting castings rich in enzymes, microbes, and plant-available nutrients. This natural fertilization process benefits the turf and soil structure. Insects like cicada killer wasps and ants also contribute to the ecosystem by acting as predators, controlling other insect populations, and contributing to soil community diversity.
Targeted Management Based on Culprit
Effective management of these mounds depends on identifying the organism responsible. If the issue is the presence of large, conical mole hills, the most effective strategy is often trapping in main runways during the active seasons of spring and fall. Another approach involves reducing their primary food source, such as grubs and earthworms, by applying beneficial nematodes or insecticides.
For the small, granular earthworm castings, there is generally no need for chemical treatment, as these creatures are beneficial to the soil. Management focuses on cultural practices, such as raking or dragging the casts when they are dry to disperse the nutrient-rich material back into the lawn. Reducing surface moisture by adjusting watering routines can also discourage earthworms from coming to the surface to cast.
If the mounds are the distinct volcano-shaped piles created by cicada killer wasps, the strategy involves making the soil less hospitable, as they prefer loose, sandy areas. Promoting a dense, healthy turf through regular watering and fertilization can deter nesting. For direct control, insecticidal dust can be applied into the burrow entrance during the evening when the wasps are inside. For crayfish chimneys, which thrive in saturated soil, the solution is habitat modification, specifically improving drainage to lower the water table. Chemical controls are not recommended due to the risk of water contamination.